This invention relates to a machine tool and more particularly to a boring or milling machine, with a measuring device for correcting the thermally-conditioned spindle displacement, which device is arranged between the spindle or its bearing and a region of the machine frame which is not influenced by the heating of the spindle.
There have been many proposals to reduce the influence of thermally-conditioned distortions in machine tools in which a very high and always constant precision is required for the machining. The chief internal sources of heat are the bearings, the gearing and the hydraulic oil, and the chief sources of external heat input are radiation heating and especially incident sunlight. To compensate such thermally-conditioned distortions, it has already been proposed to mount an expansion device in the casing of machine tools, especially for boring and milling machines, parallel to the most thermally expanding parts of the casing or parallel to those thermally expanding parts of the casing which most influence displacement of the spindle. The expansion device cooperates with a compensating expansion device and is connected to the casing by way of a connecting element such as a blade spring. With such a known arrangement, (the Deckel Company) there is achieved a compensation of the thermal expansion. However, not all the influences of temperature are compensated for. It has also already been proposed to use a measuring device of invar steel (Milwaukee-Matic), which acts on a dial gauge and an angle indicator and therewith influences the control of the machine. The use of invar rods for these has however an associated set of problems. Furthermore, since even invar steels have a thermal coefficient of expansion, a complete compensation of thermally-conditioned distortions cannot therefore be achieved.
For example, in the case of boring machines, the spindle bearing heats up when running and the casing expands. In this way inaccuracies of up to about 6/100 mm are obtained if the tool had a completely uniform temperature before the start of the operation. The heating, and thus also the expansion of the casing, comes about slowly over a long period of time. Especially in the case when bores are to be made in casings which are pre-bored and arranged in batches for final boring, the result is that the newly made bores, at the beginning of the batch, are precisely aligned when the machine is still cold but are displaced later, when the machine is warm, by up to 6/100 mm in relation to the nominal measurement.